External bone fixators were developed to enable surgeons to reestablish the alignment of bone pieces at a fracture site, and to reduce and stabilize the fracture to promote healing. Such fixators generally attach to the bone on opposite sides of the fracture
External fixators may differ both in the number of degrees of freedom, or articulations, that they provide and in the relative independence of these articulations, both mechanical and geometrical. Fixators designed to treat fractures near the centers of long bones typically have relatively few degrees of freedom or articulations. In contrast, fixators designed to treat fractures near joints typically provide many more degrees of freedom. These additional degrees of freedom are important, where there is too little room to place the pins in the fractured bone between the fracture and the joint, because alignment must be established using pins placed in a bone on the far side of the joint from the fracture. For treatment of fractures near joints that can rotate, flex, and abduct, such as the wrist, the fixator should offer some equivalent adjustment to accommodate the flexibility of the skeletal joint, so that the surgeon can establish the proper fracture alignment using forces transmitted through the joint.
Modern fixators tend to provide a large number of articulations, of various kinds. The ball joint probably is the most common articulation. A ball joint provides one rotational and two pivotal degrees of freedom. These three degrees of freedom may be fixed simultaneously using a single setscrew or other locking mechanism. Unfortunately, ball joints cannot be loosened for motion in only one degree of freedom, without being loosened to move in other degrees of freedom. Thus, a surgeon cannot loosen the ball joint slightly to pivot it a small amount in one direction, without potentially introducing changes affecting the other pivot and rotation settings.
To address these limitations, some fixators eliminate ball joints, relying instead on a combination of independent articulations to provide the necessary flexibility. The benefit of such a system is that each degree of freedom is mechanically independent of every other degree of freedom. Thus, a surgeon can adjust the position of a single articulation in the fixator without affecting the settings of other articulations. Unfortunately, a given geometric readjustment of the fractured ends of the bone(s) at a fracture site may not correspond to an adjustment of any single articulation. Instead, proper readjustment may require the surgeon to adjust several separate articulations, reducing or eliminating the benefit of independent articulations. Moreover, movement of one articulation may change the alignment of bone ends previously established by another articulation.
Articulations that have only a single degree of freedom, such as a simple pivot or slide, typically involve two basic adjustment techniques: (1) free, and (2) gear driven. Free articulations may be freely adjustable, until some type of lock is applied to secure the articulation at a selected setting. Loosening the lock allows the articulation to move relatively freely as the surgeon applies force to the joined members. In contrast, gear-driven articulations move under the control of an adjustment mechanism, such as a worm gear and rack or similar structure, which may provide mechanical advantage. For example, turning a worm gear causes the articulation to move incrementally, in accord with the rotation of the worm gear. Gear-driven articulation generally provides surgeons with greater precision and control when making fine adjustments, but it hinders rapid gross corrections. It is possible to provide an articulation with both free and gear-driven properties; however, to allow free motion of the articulation, the mechanical advantage provided by the gear reduction must be rather minimal. Unfortunately, a small mechanical advantage would reduce the precision of the adjustment, negating the very purpose for which a gear drive would be used in the first place.
Most fixators also include an extensible/contractible articulation to allow control of the longitudinal spacing between pins on opposite sides of the fracture. This type of translational freedom can be used to accommodate individuals of various sizes, as well as to distract (i.e., pull on) the fracture, if necessary. In addition, for general-purpose fixators, which are not designed for specific fractures, translational degrees of freedom can be used to create whatever spacing is required on either side of the fracture to allow for proper pin placement.
Fixators may be designed for general-purpose or fracture-specific use. General-purpose fixators typically are designed with considerable flexibility, to accommodate many different types of fractures. In contrast, fracture-specific fixators typically are designed with fewer degrees of freedom, for use on a specific type of fracture. These articulations may be tailored to correct for specific fracture displacements, and, for fractures too close to a joint to allow pin placement on both sides of the fracture, to compensate for varying joint position. Articulations corresponding to joint movements also may be used to set the joint in a comfortable position, as well as align the ends of the bone at the fracture site.
Fixators may be used to treat a variety of fractures, including Colles' fractures, which are fractures of the distal radius that usually result from falls on an outstretched hand. In Colles' fractures, the fracture line usually is quite close to the distal head of the radius, making it difficult or impossible to mount pins in the radius on the distal side of the fracture, due to a lack of space, the number of tendons and nerves in the area, and/or the typically poor bone quality. Therefore, such fractures typically are reduced using a first pair of pins set in a metacarpal bone and a second pair of pins set in the radius on the proximal side of the fracture. To reduce damage to tendons and nerves, the radial pins usually are set in the third quarter of the radius, i.e., the proximal half of the distal half of the radius. Because the pins are set on opposite side of the wrist joint, the fixator must be sufficiently articulated to reduce the fracture using forces transmitted through the wrist joint.
The wrist joint allows the hand to move in three degrees of freedom relative to the forearm. First, the hand can move in supination and pronation, i.e., rotating about the longitudinal axis of the forearm. Second, the hand can move in adduction and abduction, i.e., pivoting about an axis perpendicular to the plane of the palm. Finally, the hand can move in flexion and extension, i.e., pivoting about an axis in the plane of the palm and perpendicular to the longitudinal axis of the forearm.
Most wrist fixators are put into place to stabilize comminuted fractures, in which the bone has broken into many small pieces. In these cases, the fixator may be used to achieve and/or maintain the proper length of the broken bone. External wrist fixators generally offer significant advantages in such cases, as the fixator can apply a significant pull on the wrist without interfering with the tendons and nerves running through the wrist joint.
Unfortunately, fractures treated with external fixators may take a long time to heal. For example, in the case of wrist fractures, the external fixator may be left in place for as long as twelve weeks, followed by up to a year of physical therapy to regain strength in the injured wrist. It would be preferable to allow some degree of mobility in the joint, particularly during the latter stages of healing, so that the wrist can flex, decreasing the need for rehabilitation and shortening recovery time.
Wrist fixators incorporating one or more ball joints have been described previously, where the ball joint can be locked into position for static fixation, or released to allow limited movement of the wrist. Unfortunately, such wrist fixators typically possess several disadvantages, as described below.
First, to reduce disruption of the fracture upon releasing the ball joint, the center of the ball must be aligned precisely with the center of wrist movement, typically the palpable groove between the lunate and capitate bones. However, initial installation of the fixator may be complicated for comminuted fractures, because the wrist often is swollen, making it difficult to identify the location of the capitate-lunate junction. Improper alignment of the ball joint eventually may disrupt the fracture, extending the healing process, and potentially increasing the discomfort to the patient.
Second, the ball joints employed in previously used wrist fixators do not allow an incremental increase in freedom of movement, as discussed above. When locked, the ball joint is immobile. However, when the ball joint is unlocked, up to 90% of the range of motion of the ball joint suddenly may be restored. The sudden return of full motion may injure or at least slow the recovery of a weakened wrist.
Third, wrist fixators incorporating ball joints are unable to allow ulnar deviation (abduction of the hand) during recovery. Again, this motion is completely unavailable to the patient until the fixator is removed, at which point full mobility of the wrist is restored, potentially resulting in discomfort or injury to the weakened joint.
Thus, there is a need for a wrist fixator that could immobilize and distract a fracture of the distal radius, yet provide incremental and adjustable increases in the freedom of movement of the wrist, including supination and pronation of the hand, flexion and extension of the hand, and some degree of ulnar deviation. A preferred wrist fixator also would allow flexibility in alignment with the wrist joint, allowing a more streamlined and less demanding installation process.